Filters are used in communications systems for many purposes. In radio receivers, an active filter can be used to amplify signals at certain frequencies and reject those at other frequencies as to segregate wanted information from unwanted. In other words, filters allow the radio receiver to provide the listener with substantially the desired signal and substantially reject all other signals. In radio transmitters, filters act to mitigate extraneous emissions out of the intended band. A filter may pass the signals in the intended band and attenuate those signals that lie outside of the intended band. The radio transmitter can generate the signals in the desired band and will not transmit other signals that might interfere with other spectrum users.
Analog filters used in the baseband section of RF receivers typically satisfy stringent noise, linearity, power dissipation, and selectivity requirements. The existence of large interferers near the desired signal frequency demands a high linearity and/or low noise in the filters, impacting the distribution of gain and noise through the receiver chain.
For these reasons, on-chip filters typically use a lot of die area. A multi standard radio may support several standards, each with different bandwidth requirements. Due to differing requirements for each standard, different filters are typically used. Multiple filters further aggravate the amount of die area used to implement filters. Multi standard radios that implement one filter for each standard add the cost of having the multiple filters to the cost of the radio.
Wireless communication devices having the ability to operate under different communication standards are presently in demand. Consumers of the multi-mode devices want increased levels of performance, but also desire low cost and small physical size. It is desirable to improve the performance of a multi-mode device without increasing the cost of the device and without increasing the size of the device.